Silver-containing thermographic and photothermographic imaging materials (that is, thermally developable imaging materials) that are imaged and/or developed using heat and without liquid processing have been known in the art for many years.
Silver-containing direct thermographic imaging materials are non-photosensitive materials that are used in a recording process wherein images are generated by the direct application of thermal energy and in the absence of a processing solvent. These materials generally comprise a support having disposed thereon (a) a relatively or completely non-photosensitive source of reducible silver ions, (b) a reducing composition (acting as a black-and-white silver developer) for the reducible silver ions, and (c) a suitable binder. Thermographic materials are sometimes called “direct thermal” materials in the art because they are directly imaged by a source of thermal energy without any transfer of the image or image-forming materials to another element (such as in thermal dye transfer).
In a typical thermographic construction, the image-forming thermographic layers comprise non-photosensitive reducible silver salts of long chain fatty acids. A preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, such as behenic acid or mixtures of acids of similar molecular weight. At elevated temperatures, the silver of the silver carboxylate is reduced by a reducing agent for silver ion (also known as a developer), whereby an image of elemental silver is formed. Preferred reducing agents include methyl gallate, hydroquinone, substituted-hydroquinones, hindered phenols, catechols, pyrogallol, ascorbic acid, and ascorbic acid derivatives.
Some thermographic constructions are imaged by contacting them with the thermal head of a thermographic recording apparatus such as a thermal print-head of a thermal printer or thermal facsimile. In such constructions, an anti-stick layer is coated on top of the imaging layer to prevent sticking of the thermographic construction to the thermal head of the apparatus used. The resulting thermographic construction is then heated imagewise to an elevated temperature, typically in the range of from about 60 to about 225° C., resulting in the formation of a black-and-white image.
Silver-containing photothermographic imaging materials (that is, photosensitive thermally developable imaging materials) that are imaged with actinic radiation and then developed using heat and without liquid processing, have also been known in the art for many years. Such materials are used in a recording process wherein an image is formed by imagewise exposure of the photothermographic material to specific electromagnetic radiation (for example, X-radiation, or ultraviolet, visible, or infrared radiation) and developed by the use of thermal energy. These materials, also known as “dry silver” materials, generally comprise a support having coated thereon: (a) a photocatalyst (that is, a photosensitive compound such as silver halide) that upon such exposure provides a latent image in exposed grains that are capable of acting as a catalyst for the subsequent formation of a silver image in a development step, (b) a relatively or completely non-photosensitive source of reducible silver ions, (c) a reducing composition (acting as a developer) for the reducible silver ions, and (d) a binder. The latent image is then developed by application of thermal energy.
In photothermographic materials, exposure of the photosensitive silver halide to light produces small clusters containing silver atoms (Ag0)n. The imagewise distribution of these clusters, known in the art as a latent image, is generally not visible by ordinary means. Thus, the photosensitive material must be further developed to produce a visible image. This is accomplished by the reduction of silver ions that are in catalytic proximity to silver halide grains bearing the silver-containing clusters of the latent image. This produces a black-and-white image. The non-photosensitive silver source is catalytically reduced to form the visible black-and-white negative image of silver while much of the silver halide, generally, remains as silver halide and is not reduced.
In photothermographic materials, a typical non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, such as behenic acid or mixtures of acids of similar molecular weight. At elevated temperatures, the silver of the silver carboxylate is reduced by a reducing agent for silver ion (also known as a developer), whereby elemental silver is formed. The reducing agent for the reducible silver ions, often referred to as a “developer”, may be any compound that, in the presence of the latent image, can reduce silver ion to metallic silver and is usually of relatively low activity until it is heated to a temperature sufficient to cause the reaction. A wide variety of classes of compounds have been disclosed in the literature that function as reducing agents for photothermographic materials. Upon heating, and at elevated temperatures, the reducible silver ions are reduced by the reducing agent. This reaction occurs preferentially in the regions surrounding the latent image and produces a negative image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the photothermographic imaging layer(s).
Differences Between Photothermography and Photography:
The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing with aqueous processing solutions.
In photothermographic imaging materials, a visible image is created in the absence of a processing solvent by heat as a result of the reaction of a reducing agent incorporated within the material. Heating at 50° C. or more is essential for this dry development. In contrast, conventional photographic imaging materials require processing in aqueous processing baths at more moderate temperatures (from 30° C. to 50° C.) to provide a visible image.
In photothermographic materials, only a small amount of silver halide is used to capture light and a non-photosensitive source of reducible silver ions (for example, a silver carboxylate or a silver benzotriazole) is used to generate the visible image using thermal development. Thus, the imaged photosensitive silver halide serves as a catalyst for the physical development process involving the non-photosensitive source of reducible silver ions and the incorporated reducing agent. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver (that is, silver halide) that, upon chemical development, is itself at least partially converted into the silver image, or that upon physical development requires addition of an external silver source (or other reducible metal ions that form black images upon reduction to the corresponding metal). Thus, photothermographic materials require an amount of silver halide per unit area that is only a fraction of that used in conventional wet-processed photographic materials.
In photothermographic materials, all of the “chemistry” for imaging is incorporated within the material itself. For example, such materials include a reducing agent (that is, a developer for the reducible silver ions) while conventional photographic materials usually do not. The incorporation of the reducing agent into photothermographic materials can lead to increased formation of various types of “fog” or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems.
Moreover, in photothermographic materials, the unexposed silver halide generally remains intact after development and the material must be stabilized against further imaging and development. In contrast, silver halide is removed from conventional photographic materials after solution development to prevent further imaging (that is in the aqueous fixing step).
Because photothermographic materials require dry thermal processing, they present distinctly different problems and require different materials in manufacture and use, compared to conventional, wet-processed silver halide photographic materials. Additives that have one effect in conventional silver halide photographic materials may behave quite differently when incorporated in photothermographic materials where the underlying chemistry is significantly more complex. The incorporation of such additives as, for example, stabilizers, antifoggants, speed enhancers, supersensitizers, and spectral and chemical sensitizers in conventional photographic materials is not predictive of whether such additives will prove beneficial or detrimental in photothermographic materials. For example, it is not uncommon for a photographic antifoggant useful in conventional photographic materials to cause various types of fog when incorporated into photothermographic materials, or for supersensitizers that are effective in photographic materials to be inactive in photothermographic materials.
These and other distinctions between photothermographic and photographic materials are described in Unconventional Imaging Processes, E. Brinckman et al. (Eds.), The Focal Press, London and New York, 1978, pp. 74-75, in D. H. Klosterboer, Imaging Processes and Materials, (Neblette's Eighth Edition), J. Sturge, V. Walworth, and A. Shepp, Eds., Van Nostrand-Reinhold, New York, 1989, Chapter 9, pp. 279-291, in C. Zou et al., J. Imaging Sci. Technol. 1996, 40, pp. 94-103, and in M. R. V. Sahyun, J. Imaging Sci. Technol. 1998, 42, 23.
Problem to be Solved:
Organic solvent-based photothermographic materials typically consist of a substrate, onto which is coated an imaging layer comprising a photothermographic emulsion in a polyvinyl butyral binder. A protective overcoat often is coated on top of the emulsion layer. These protective overcoats typically comprise predominately cellulose acetate butyrate (CAB) polymers. [See for example, U.S. Pat. No. 6,368,778 (Kong et al.)]. While adequate for some purposes, CAB is somewhat soft and its use can lead to sticking, scratching, and marring during feeding, imaging, and particularly during thermal development of thermally developable materials. Addition of a small amount of a polymethyl methacrylate provides good adhesion to the thermally developable emulsion layer, but the resultant overcoat is still soft, and sticking, scratching, and marring during various stages of imaging and development is still a problem.
U.S. Pat. No. 2,785,993 (Paist et al.) describes a trilayer construction of cellulose acetate, coated onto methyl vinyl ether/maleic anhydride copolymer, coated onto polyvinyl butyral, coated on an aluminum sheet. This construction is said to afford good adhesion of all of the layers and of the polyvinyl butyral to the aluminum. The constructions are said to be useful for printing plates and decorative articles. No mention is made of mixtures of vinyl ether/maleic anhydride copolymers with cellulose acetate. No photographic, thermographic, or photothermographic materials are in the polyvinyl butyral layer. The various layers are not coated simultaneously.
U.S. Pat. No. 4,741,992 (Przezdziecki) describes the use of poly(silicic acid) and water-soluble hydroxyl-containing polymers as an overcoat layer having good adhesion to polyvinyl butyral for use in thermally developable materials.
U.S. Pat. No. 5,804,365 (Bauer et al.) describes the addition of small amounts of organic non-ionic boron compounds to imaging layers containing polyvinyl acetal binders. Such addition is said to cross-link the binder and improve adhesion of polyvinyl alcohol overcoat layers.
U.S. Pat. No. 5,891,610 (Bauer et al.) describes the use of poly(silicic acid), a water-soluble hydroxyl-containing polymer, and a water-soluble polyvinyl acetal as a protective topcoat having good adhesion to polyvinyl butyral for use in thermally developable materials.
U.S. Pat. No. 5,928,857 (Geisler et al.) describes the addition of certain adhesion promoting resins to the emulsion layer to promote adhesion of the emulsion layer to the support.
Cellulose acetate (CA) has been used as the sole binder in protective overcoats for photothermographic materials. [See for example, U.S. Pat. No. 3,933,508 (Ohkubo et al.)]. Although cellulose acetate is harder and has a higher softening temperature than CAB, its use as protective overcoat for polyvinyl butyral based photothermographic materials creates a problem due to the poor adhesion of cellulose acetate to polyvinyl butyral. Addition of polymethyl methacrylate does not work in this construction, as polymethyl methacrylate does not promote the adhesion of cellulose acetate to polyvinyl butyral. The two materials are incompatible and produce hazy, non-uniform coatings.
U.S. Pat. No. 4,452,883 (Frenchik et al.) describes the use of mixtures of polyvinylpyrrolidine and methyl vinyl ether/maleic anhydride copolymer as barrier polymers for color photothermographic materials.
A need exists for improved protective overcoats for thermally developable materials that exhibit good adhesion to the photothermographic emulsion layer, are harder than the currently used overcoats, and have good optical clarity, with little if any effect on sensitometric properties.